A highway operator often wishes to gather information about vehicles using the highway. The speeds and journey times of vehicles are particularly of interest. For example, the operator of a motorway from London to Bristol may wish to know the speed of individual vehicles at one or a number of locations. The instantaneous speeds of vehicles at predefined locations are known as “spot speeds”. The operator may also wish to know the average travel time between London and Bristol, for example, or for sections of the route. This travel time can be estimated from the spot speeds measured at the measurement points. The methods to integrate the journey time from the spot speeds are well known and will not be described herein.
There are two roadside systems in general use for measuring the speed of vehicles at a particular location: One of these uses two sensors a fixed distance apart. Sensors can for example take the form of light beams arranged to be broken by passing vehicles, or electromagnetic coils or pressure sensors buried in the roadway. The time taken for a vehicle to pass from one sensor to the other is measured, and the speed of the vehicle can be calculated from this “time of flight”. Roadside systems that use such a system have the problem that over time they may drift out of calibration.
Another roadside system in general use takes advantage of the Doppler effect. A radar source is directed towards oncoming traffic, and radio waves reflected back towards the source from the moving traffic are detected. The speed of a vehicle travelling towards such a radar source can be calculated from the change in frequency of the radio waves reflected from that vehicle. Such systems are unlikely to drift out of calibration over time. However, systems with Doppler radar are subject to installation and orientation errors that introduce the “cosine effect” whereby all speeds of vehicles are under-read by a certain proportion, determined by the angle of the radar beam relative to the vehicle direction.
Before the results of spot speed measurement can be used for analysis of the traffic stream, the accuracy of each measurement station needs to be assessed. Final results are not useful unless a confidence limit can be determined for the spot speed of all vehicles at each site and the average speeds for all vehicles at a selection of sites constituting a journey. Furthermore, measurement stations need to be assessed for accuracy at regular time intervals following their initial installation, to confirm that they have not drifted away from calibration. Typically, measurement stations need to be assessed approximately every three months.
The equipment and method for assessing measurement stations needs to be suitable for fast and efficient verification of speed monitoring equipment. This means that the system must be portable and suitable for quick deployment or assessment.
At present, systems for speed measurement assessment include the following methods:                Radar (Doppler) or LIDAR (Laser Diode Ranging).        Two light beams horizontally or vertically across the carriageway.        Two pressure sensors on the road surface.        
Radar devices use the Doppler effect as described above. When portable devices are used, the radio source and receiver are located in a hand held device (a “speed gun”). Such devices are very accurate when used in suitable conditions, but can still give rise to a number of drawbacks. Firstly, when a motorist sees a speed gun in use, they will often apply the brakes, or at least take their foot off the accelerator. This means that the vehicle will be slowing as it passes the sensor and this will introduce a measurement error. Furthermore, the method is very labour-intensive and difficult to use in heavy traffic. There are errors introduced by the “cosine” effect, the effect of the angle between the gun beam and the vehicle direction.
Two horizontal light beams or pressure sensors on the road surface may be used successfully in low volume single lane carriageways. However, many modern roads are dense dual carriageways, and these methods are impractical in practice. Installing sensors on the road is hazardous and can easily lead to an accident.
Thus the present methods for assessing the accuracy of road-side measurement are relatively inefficient, inaccurate and can be unsafe to use.